qemu: implement driver's shutdown/shutdown wait methods

qemu: implement driver's shutdown/shutdown wait methods

On shutdown we just stop accepting new jobs for worker thread so that on
shutdown wait we can exit worker thread faster. Yes we basically stop
processing of events for VMs but we are going to do so anyway in case of daemon
shutdown.

At the same time synchronous event processing that some API calls may require
are still possible as per VM event loop is still running and we don't need
worker thread for synchronous event processing.

Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>